Some tweaks in libsimpleservo C API

[paulrouget]

• Adding logs.
• Adding OpenGL support for OSX.
• Fixing file reading issue.

With this, I can load Servo into a C app (with the C version of libui + andlabs/libui#405).

---

This change is 

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[jdm]

@bors-servo r+

[bors-servo]

📌 Commit a206a80 has been approved by jdm

[bors-servo]

⌛ Testing commit a206a80 with merge df20520...

[bors-servo]

💔 Test failed - android-mac

[paulrouget]

I removed the commit that changes how file content is read. Hopefully we won't need this if this PR lands: #22836

[paulrouget]

@bors-servo r=jdm

[bors-servo]

📌 Commit 6060a36 has been approved by jdm

[bors-servo]

⌛ Testing commit 6060a36 with merge 7e57f1b...

[bors-servo]

💔 Test failed - status-taskcluster

[jdm]

error[E0412]: cannot find type `Rc` in this scope
  --> ports/libsimpleservo/api/src/gl_glue.rs:82:29
   |
82 |     pub fn init() -> Result<Rc<Gl>, &'static str> {
   |                             ^^ not found in this scope
help: possible candidate is found in another module, you can import it into scope
   |
82 |     use std::rc::Rc;
   |
error[E0412]: cannot find type `Gl` in this scope
  --> ports/libsimpleservo/api/src/gl_glue.rs:82:32
   |
82 |     pub fn init() -> Result<Rc<Gl>, &'static str> {
   |                                ^^ not found in this scope
help: possible candidate is found in another module, you can import it into scope
   |
82 |     use servo::gl::Gl;
   |
error: aborting due to 2 previous errors

[bors-servo]

☔ The latest upstream changes (presumably #22836) made this pull request unmergeable. Please resolve the merge conflicts.

[paulrouget]

@bors-servo r=jdm

[bors-servo]

📌 Commit 1292db6 has been approved by jdm

[bors-servo]

⌛ Testing commit 1292db6 with merge d029b1a...

[bors-servo]

☀️ Test successful - android-mac, arm32, arm64, linux-rel-css, linux-rel-wpt, mac-rel-css1, mac-rel-css2, mac-rel-wpt1, mac-rel-wpt2, mac-rel-wpt3, mac-rel-wpt4, magicleap, status-taskcluster
Approved by: jdm
Pushing d029b1a to master...